Relatively little is known about the genetic regulation of plant root development and function. Elucidation of the genetic regulation is important because roots serve important functions such as acquisition of water and nutrients and the anchorage of the plants in the soil.
Maize root architecture is composed of different root types formed at different plant developmental stages. A number of mutants affected in specific root types during different developmental stages have been described in maize (e.g. rtcs (rootless concerning crown and seminal roots), Irt1 (lateral rootless1)). The monogenic recessive rum1 ((rootless with undetectable meristems 1) mutant was first reported by Woll et al. (2004) Maize Genetics Cooperation Newsletter 78: 59-60. A more detailed description of the mutant phenotype was published by Woll et al. (2005) Plant Physiology 139 (3): 1255-1267. The maize mutant was shown to be impaired in the formation of seminal and lateral roots on the primary root. No obvious differences were detectable in aboveground development between rum1 and wild-type plants. Genetic analysis of the rum1 mutation indicated that it is inherited as a monogenic recessive trait. However, introduction of the rum1 mutation into different genetic backgrounds resulted in segregation ratios that suggested the presence of a recessive suppressor of the rum1 mutation in those backgrounds.
The plant hormone auxin plays a crucial role during embryogenesis and is involved in various aspects of root development. In the rum1 mutant, auxin transport toward the root tip is severely reduced. Mutations in members of the auxin-inducible Aux/IAA and ARF gene families of Arabidopsis result in phenotypes that resemble the maize rum1 phenotype in regard to the absence of lateral roots on the primary root. Several gain-of-function mutants lacking lateral roots or inhibited in lateral root formation have been described in Arabidopsis (Solitary-Root/IAA14 gene (SLR/IAA14) described by Fukaki et al. (2002) The Plant Journal 29(2): 153-168; Massugu2/IAA19 gene (MSG2/IAA19) described by Tatematsu et al. (2004) Plant Cell 16: 379-393. Okushima et al. (2005) Plant Cell 17: 444-463 described a arf7arf19 double mutant, that shows a phenotype similar to the slr/iaa14 and msg/iaa19 mutants.
In vitro experiments indicate that IAA14 interacts with both ARF7 and ARF19, and that IAA19 interacts with ARF7. Aux/IAA and ARFs are therefore considered major components of the auxin signaling pathway that controls plant growth responses to the hormone auxin.
Despite the extensive genetic and morphological characterization of the rum1 mutant, there has been no molecular analysis of the nucleic acid encoding the protein associated with the rum1 phenotype. Indeed, the identity of the protein encoded by rum1 has not been reported.